Distributed optical fibre sensor

ABSTRACT

There is described a distributed optical fiber sensor for detecting one or more physical parameters indicative of an environmental influence on a sensor optical fiber, as a function of position along the sensor fiber. The sensor uses probe light pulses of different wavelengths. At least some of the probe light pulses may also be of different pulse lengths. The relative phase bias between interferometric signals in backscattered probe light of different wavelength pulses may also be controlled.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/884,017, filed on May 8, 2013, which is a national phase applicationunder 35 U.S.C. 371 of International Application No. PCT/GB2011/052183,filed on Nov. 9, 2011, which claims priority to and the benefit of U.K.Patent Application No. 1019117.9, filed on Nov. 11, 2010, the entiredisclosures of each of which are incorporated by reference herein.

The present invention relates to distributed optical fibre sensors, inwhich one or more physical parameters are sensed as a function ofposition along a sensing optical fibre from the properties of probelight backscattered within the sensing fibre. In particular, but notexclusively, the invention relates to optical time domain reflectometry(OTDR) sensors for use in sensing vibration, and such sensors which usephase sensitive OTDR techniques such as through the detection ofcoherent Rayleigh noise, or other interferometric techniques.

INTRODUCTION

Distributed optical fibre sensing is a well known approach to providinginformation about environmental conditions surrounding a sensing opticalfibre. Fully-distributed sensing in principle provides spatiallyresolved information from every point along the fibre. Variables thatcan be sensed include temperature, static strain, pressure, andvibration.

One such technique detects variations in refractive index, induced by aphysical forcing such as vibration, in the coherent Rayleigh noiseprofile of light backscattered within a sensing optical fibreinterrogated by an optical source of limited bandwidth. Such Rayleighnoise profiles arise from interference between the many components ofthe backscattered light originating from different points along aportion of the sensing optical fibre illuminated by the optical source.Such techniques are described, for example, in WO2008/056143.

It would be desirable to address problems and limitations of the relatedprior art.

SUMMARY OF THE INVENTION

Accordingly, the invention provides a distributed optical fibre sensorfor determining at least one parameter, typically a physical parameter,from properties of probe light backscattered within the sensing fibre,the sensor comprising: a probe light source arranged to launch probelight pulses into the sensing fibre, the probe light pulses includinggroups each of two or more probe light pulses having different opticalwavelengths to each other; a detector arranged to detect probe lightbackscattered in said sensing fibre, including separately detectinglight of each of said different wavelengths; and an analyser arranged todetermine said parameter as a function of position along the sensingfibre from at least some of said detected backscattered probe light, orfrom the detected backscattered probe light of at least some of thepulses of the group.

In particular, the parameter may be a parameter of the environmentaround the sensing fibre, such as static pressure, temperature,vibration, or strain on the fibre exerted by the environment, and towhich the fibre is responsive.

Such apparatus provides a number of benefits, depending on how the probelight pulses are controlled, detected and analysed. For example, animportant performance feature of some distributed optical fibre sensors,such as vibrational sensors, is the maximum frequency of detection orresponse. In optical fibre distributed sensing systems where theinterrogation of the sensing fibre is arranged to provide spatialmapping in the time domain, the maximum frequency of response is limitedby the round-trip time required for light to travel at least oncebetween the two ends of the sensing fibre. For very long sensing fibres,this limitation can become troublesome.

The backscattered light from each pulse of a group, and therefore ofeach different wavelength, may be detected using a separatephotodetector, such as a separate photodiode. Particular embodiments mayuse three, four, or more different optical wavelengths in a group ofpulses, and may use a corresponding number of photodetectors.

The pulses of each group may be launched such that at leastbackscattered light from all the pulses of each group coexists withinthe sensing fibre, and the detector may then both separately andsimultaneously detect light of each of said different wavelengths.

Accordingly, the probe light source may be arranged to interleavelaunching of the probe light pulses of different wavelengths, and theanalyser may be arranged to construct, for a plurality of positionsalong the sensing fibre, time series of said physical parameter byinterleaving data derived from the detected backscatter of probe lightfor each of said different wavelengths.

Using this technique, multiple probe light pulses can be launched andthe corresponding backscattered light from each pulse separatelydetected within the time frame in which only a single probe light pulseof a single wavelength can be used. An acoustic sensor can therefore beprovided with an enhanced acoustic range extending to higher frequenciesthan a corresponding sensor using a single probe wavelength.

The apparatus may be arranged, for example, to launch said probe lightpulses such that interference signals from each wavelength can bedetected in the backscattered light, to detect said interference signalsin the backscattered light of each optical wavelength, and to determinesaid parameter as a function of position along the sensing fibre fromsaid detected interference signals.

The intensity of light reflected back toward the illuminated end of thesensing fibre in various types of distributed fibre optic sensors, andin particular sensors using coherent Rayleigh noise or otherinterferometric techniques, is not intrinsically a linear function ofthe instantaneous amplitude of the physical parameter to be detected.Rather, the response characteristics for a linear change in the localforcing, such as a vibration-induced strain of the sensing fibre, isperiodic and of variable sign and scale factor. This occurs because ofthe interferometric nature of the optical response. The optical responseof the backscattered light to changing strain is governed by aproportional change in relative phase imbalance between parts of a probelight pulse reflected from different positions along the fibre. The netresult of the changing phase on the sum of all of the backscatteredcomponents is typically to generate a sinusoidal variation ofbackscattered intensity as the relative phase is changed linearly.Moreover, since the starting point of the intensity change may be at anypoint within the sinusoidal response, a given fractional change invibrational strain can generate either a positive or negative fractionalchange in intensity, and with a scale factor that can vary between zeroand some maximum value that depends on the interferometric fringevisibility within the optical system. Since the lower limit ofvibrational strain that can be detected by the system is also limited bysystem noise arising from various sources, the dynamic range of theresponse of the system is constrained to lie within a certain range ofvibration amplitude, and within an even more restricted range if alinear response is desired. A linear response is often desirable.

Accordingly, the probe light source may be arranged to launch some orall of the probe light pulses of a group with different pulse durations.For these purposes, the pulses of a group need not have differentwavelengths, for example if two or more of the pulses are launched atsufficient spacing that they can be separately detected when of the samewavelength. The magnitude of the phase imbalance experienced by a probelight pulse for a particular physical forcing, and therefore the scalingof the response characteristics of the sensor, is dependent upon thelength of the probe light pulse. By launching probe light pulses ofdifferent durations, a range of response characteristics can be achievedat the same time. In particular, the pulses of different duration may belaunched within a time frame so as to coexist within the fibre, or forsome applications much closer together, for example so as to overlap, orspaced so as not to coexist within the fibre.

One application of this technique is for the analyser to be arranged tocompare between data derived from the detected backscatter of the pulsesof different pulse durations in the group. The analyser can then also bearranged to derive a measure of non-linearity of response characteristicfor each pulse duration. The sensor may then be arranged to adjustdurations of launched probe light pulses dependent upon the results ofsaid comparison. Using such a scheme, the sensor can adaptively providea more linear response across particular sections or the whole of thesensing fibre, for example by selecting a pulse duration for use indetermining the physical parameter which gives the optimum signal tonoise ratio without excessive non-linearity.

In another application of this technique the analyser is adapted toweight the use of data derived from the detected backscatter of thepulses to preferentially select for data from a subset of the differentpulse durations. For example, data from pulse durations wherenon-linearity is evident may be rejected or be used with a lowweighting. Data from pulse durations with a level of non-linearity belowa certain threshold may be preferentially or exclusively used. Data withthe maximum pulse length duration where the level of non-linearity isstill below the threshold may be preferentially or exclusively used, inorder to select for the data with the strongest response characteristicswithout being unduly compromised by non-linearity.

Data from different pulse lengths or durations may be selected fordifferent positions or regions along the sensing fibre, even from withinpulses of the same group. In this way, the dynamic range of the sensorcan be optimised for local conditions such as areas with high levels andlow levels of noise.

Generally, the local phase imbalance at any point along the sensingfibre is highly sensitive to the wavelength of the probe light pulse.However, it is observed that by careful control of the probe lightwavelength, the phase biases in the interferometric backscatter signalsarising from pulses of different wavelengths can be aligned or broughtinto a particular spacing, and that these relative phase biases betweenthe different wavelengths can be maintained along much or all of thesensing fibre. As a result, the relationship between the responsecharacteristics for pulses of different wavelengths at any point alongthe sensing fibre can be controlled. The relative phase bias can beobserved from backscattered probe pulse light, and then can be adjustedaccordingly by fine adjustment of one or more of the probe lightwavelengths. In particular, such techniques apply where the sensor is aninterferometric sensor, and determines said physical parameter fromdetected interference properties of the backscattered probe light ateach probe light wavelength.

Accordingly, the sensor may be arranged to control the relative phasebias or biases between interferometric backscatter signals fromrespective pulses in each group, for example to maintain predeterminedspacings between the phase biases of different wavelengths. For example,setting the relative phase bias between the interferometric backscattersignals from two or more pulses in a group to approximately zero or πradians (parallel or antiparallel), can be used to obtain similarresponse characteristics between the pulses. If the relative phase biasfor two particular wavelengths is set to a predetermined or controlledvalue of approximately π/2 radians (orthogonal), the signals from thetwo wavelengths can be added in quadrature, or added vectorially basedon the relative phase bias, to derive a signal with a responsecharacteristic which is close to constant along the sensing fibre, or atleast which varies much less than the response characteristic from anysingle pulse.

To control the relative phase bias, the sensor may compare between dataderived from the detected backscatter of the pulses of differentwavelengths in the group, and adjust one or more of said plurality ofdifferent wavelengths dependent upon results of said comparison, so asto control the relative phase bias.

In order to assist in detection of the phase bias, the sensor mayfurther comprise a phase bias calibrator arranged to exert apredetermined forcing on a reference section of optical fibre into whichthe probe light pulses are launched. The reference section may, inparticular, be a section of the sensing fibre itself. The sensor maythen be arranged to compare between data derived from the detectedbackscatter of the pulses of different wavelength in the group, whichoccurs in said reference section of optical fibre. If the forcing is ata particular acoustic frequency then the relative phase bias or biasesof the interferometric backscatter from pulses in the group within thereference section of fibre will be evident from the phases or waveformsof the acoustic frequency signals detected from the backscattered lightin respect of each pulse wavelength.

It will be understood from the above that not all of the pulses of eachgroup need be used to determine the parameter to be sensed by thesensor. For example, if multiple pulse durations are being used, onlypulses with a satisfactory linearity of response might be used fordetermining the parameter.

The invention also provides methods corresponding to the variousapparatus features discussed above, for example of operating adistributed optical fibre sensor to determine at least one parameter asa function of position along a sensing fibre from properties of probelight backscattered within the sensing fibre, the method comprising:

launching probe light pulses into the sensing fibre, the probe lightpulses including successive groups each of two or more probe lightpulses having different optical wavelengths and/or different pulselengths to each other;

detecting probe light backscattered in said sensing fibre, includingseparately detecting light of each of said different wavelengths and/orpulse lengths; and

determining said parameter as a function of position along the sensingfibre from said detected backscattered probe light.

Some of the operational principles of the invention can be illustratedwith reference to an ideal two-path interferometer. This case representsa considerable simplification compared to practical cases, but willserve to explain the basic principles with the best clarity. Therelative phase, Δφ_(b), of light emerging from an undisturbed, two-pathinterferometer illuminated by a coherent source of fixed wavelength λ isgiven by:Δφ_(b)=2πn _(e) d/λ  (1)where n_(e) is the effective refractive index of the propagation mediumand d is the physical distance of the path imbalance in theinterferometer. When the interferometer is undisturbed, this phase iscommonly referred to as the ‘phase bias delay’ of the interferometer.Small disturbances of the interferometer will lead to correspondingdisturbances of the phase imbalance around this phase bias value. Theoutput optical intensity of an ideal interferometer with perfectlycoherent illumination is proportional to one plus the cosine of theinstantaneous phase imbalance. In the context of the invention describedhere, the location and mean path imbalance of the interferometer are setby parameters of the optical reflectometry system. In the case ofoptical time domain reflectometry, the location of the interferometer isdefined by the time delay after launching an optical interrogation pulseinto a sensing fibre, and the mean path imbalance is related to thephysical length of the portion of the fibre that is illuminated at anyinstant in time. However, the actual bias phase for each sensinglocation along the fibre will typically be very large compared to 2πradians, and the exact value of the phase bias within a range of 2π willbe unpredictable, since the fibre properties and state of backgroundstrain are prone to vary with position.

Embodiments of the invention are arranged to interrogate the sensingfibre with probe pulses of more than one optical wavelength, with thereflectometric parameters adjusted differently for each wavelength. Theinvention may be applied to optical time domain reflectometry, thoughalternative reflectometric sensing techniques could also be used andcould also have been chosen for illustrations.

BRIEF SUMMARY OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings of which:

FIG. 1 illustrates a distributed optical fibre sensor using probe lightpulses of more than one wavelength;

FIGS. 2A to 2H show some ways in which probe light pulses of FIG. 1 maybe launched relative to each other;

FIG. 3 illustrates a method for enhancing the detection rate of thesensor of FIG. 1 using interleaving;

FIGS. 4A and 4B illustrate aspects of phase imbalance and responsecharacteristics in an interferometer analogous to the sensor of claim 1;

FIG. 5 shows some details of the sensor of FIG. 1 configured to controlpulse length and therefore sensor response characteristics;

FIG. 6 illustrates use of an ensemble of pulse lengths to increasesensor dynamic range;

FIG. 7 shows some details of the sensor of FIG. 1 configured toimplement the ensemble scheme of FIG. 6;

FIGS. 8a to 8c show different arrangements of phase imbalance in aninterferometer illustrating how phase bias between interference fringesfrom pulses of different wavelengths may be controlled and used in asensor such as that of FIG. 1;

FIG. 9 shows some details of the sensor of FIG. 1 configured to controlrelative phase bias between interference fringes from pulses ofdifferent wavelengths; and

FIG. 10 shows some details of further optical and control componentswhich may be used in implementing a sensor such as that of FIG. 1.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to FIG. 1 there is illustrated a distributed optical fibresensor suitable for sensing one or more physical parameters as afunction of position along part or all of the sensing optical fibre 10,using time domain reflectometry. The sensor includes the sensing fibre10, a probe light source 12 for launching probe light pulses into thesensing fibre, a detector 14 for detecting probe light which has beenbackscattered within the sensing fibre 10, and an analyser 16 forprocessing data received from the detector.

The analyser 16 outputs analysis results such as a determination of theone or more physical parameters, and in FIG. 1 this output is passed toa computer display 18, although various other types of output mechanismmay be used. The analyser 16 also uses data derived from the detectedbackscatter to provide control signals 20 to the probe light source 12.A variety of control signals may be provided, some of which arediscussed below, including signals controlling the duration of probelight pulses and signals controlling the wavelengths of the probe lightpulses. In alternative embodiments, the control functions may beimplemented separately to the analyser 16, for example in a controllerelement. The skilled person will appreciate that the optical, electronicand data processing functionality of the sensor can be implemented anddistributed across different functional or physical units in variousways according to convenience and implementation objectives.

The probe light source 12 is arranged to launch into the sensing fibregroups of two or more probe light pulses 22, 24. Each pulse of a groupis of a different optical wavelength, shown in FIG. 1 as λ₁ and λ₂.Depending on the mode of operation of the sensor, the probe light pulsesof a group may be launched either together or at different times, sothey may propagate together or spaced from each other along the sensingfibre. There may be two, three, four or more pulses, and thereforedifferent wavelengths, in a group.

Each probe light pulse is backscattered as it propagates along thesensing fibre, as illustrated in FIG. 1 by backscattered light portions25, 26 of wavelengths λ₁ and λ₂ respectively.

Backscattered probe light is received at the detector 14. The detectoris arranged to receive and detect light of the different opticalwavelengths λ₁ and λ₂ separately. Noting that even when probe lightpulses of a group are launched in a widely spaced configuration as shownin FIG. 1, and at up to an equivalent distance spacing of twice thelength of the sensing fibre 10 (to allow for round-trip paths),backscattered light from all the pulses of a group will overlap at thedetector to some extent, the detector may also be arranged to detectlight of the different wavelengths simultaneously. The detector recordstime series data of intensity, power or other characteristics of thereceived light of each different frequency, and this data is passed tothe analyser 16.

The probe light source 12 can be implemented using one or more lasers.If two or more separate lasers are used, for example to generate probelight pulses of two or more different wavelengths for a group of probelight pulses, then at least some of the optical components such asoptical amplifiers and filters for conditioning the probe light pulsesand directing them to the sensing fibre 10 may be shared. The detector14 may similarly use separate photodetector components, such as PINdiodes, for each different wavelength of a group of probe light pulses,but may share other optical components such as optical amplifiers andfilters before the different wavelengths are demultiplexed to thephotodetector components. If the probe light pulses of differentwavelengths are launched with sufficient spacing that backscatteredlight of the different wavelengths does not overlap significantly at thedetector, then a single photodetector element may be used for allwavelengths. A more detailed description of a suitable arrangement forputting into effect the optical and electronic aspects of the sensor ofFIG. 1 is discussed below in connection with FIG. 10. Aspects of dataprocessing and control which can be implemented are illustrated inothers of the figures.

Typical wavelengths for the pulses may be around 1528 to 1562 nm. Pulseshapes may be rectangular, or of more complex shape, for example usingthe double peak form discussed in NO 2006/048647. Pulse lengths maytypically be in the range from around 1 ns to 1000 ns. Pulse grouprepetition rates may typically be between about 1 kHz and 1 MHz. Pulsepeak powers may typically be between about 0.1 and 1 W. Advantageously,the wavelengths of all the pulses in a group may be closely spaced, forexample within a wavelength band of 1 nanometer. The optical sourcebandwidth may be between about 1 kHz and 10 GHz, although coherence atthe lower end of this scale will be dominated by the frequency transformof the pulse shape. The sensing fibre is preferably of a single modetype.

In FIGS. 2A to 2H the sensor of FIG. 1 is illustrated in use withseveral different probe light pulse launching schemes, each representinguse of a different group of probe light pulses. In each figure, thesensing fibre 10 is shown by a solid line, with the distal end 28 of thesensing fibre marked in FIG. 2A, followed by a broken line 29illustrating the fullest length of the return path for backscatteredlight along the sensing fibre 10 to the detector, but for clarity ofillustration extending without reversal of direction. Probe light pulsesshown alongside, or in FIG. 2A beyond the left hand end of the brokenline 29 no longer exist, having already reached the end 28 of thesensing fibre 10, but their position in each figure is illustrative oflaunch time relative to other probe light pulses shown.

FIG. 2A shows a probe light pulse launch scheme commonly found in priorart distributed optical fibre sensors. Each pulse is of the same opticalwavelength λ₀, and one pulse follows another at sufficient time delaythat backscatter from a previous pulse all arrives at the detector 14before the next pulse is launched. This avoids weak backscatter from aprevious pulse at the distal end of the sensing fibre from mixing withstrong backscatter from a subsequent pulse newly launched into thesensing fibre, which would make interpretation of the mixed backscatterfrom the previous pulse impossible to use, and would increase noise inthe backscatter from the newly launched pulse.

In FIGS. 2B, 2C and 2D, two or more pulses of a group are launched atdifferent times so that they propagate along the sensing fibre in aspaced configuration. Unlike the arrangement of FIG. 2A, the last pulseof each group in these figures is launched before backscattered lightfrom the first pulse of the group finishes arriving at the detector, sothat backscattered light from all the pulses in the group coexists inthe sensing fibre at some point in time. Whereas in FIG. 2B the pulse λ₂is launched when nearly all of the backscatter of pulse λ₁ has returnedto the detector, in FIGS. 2C and 2D the groups of λ₁ and λ₂ pulses, orλ₁, λ₂ and λ₃ pulses may be repeated to form a series of regular andevenly spaced probe light pulses. Repetition of a group of probe lightpulses as shown in FIG. 2B, 2C or 2D also provides interleaving of probelight pulses of the different wavelengths, which can be used to increasethe rate at which the sensor interrogates the sensing fibre 10.

In FIGS. 2E and 2F the pulses of each group are launched at the same ordifferent times, but at least such that the last pulse of a group islaunched before the first pulse reaches the end 28 of the sensing fibre.In this way, all the probe light pulses of a group coexist in thesensing fibre at some point in time. FIG. 2E illustrates a group withtwo pulses launched at the same time, overlapping in physical extent, orat least close together. In FIG. 2F a group with three pulses islaunched with a spacing which is much smaller than the length of thesensor fibre, for example with a total spacing of less than 30% of thelength of the sensing fibre is shown. Depending on the application,different constraints may be applied as to how close together pulses ofa group should be launched.

In FIG. 2G the pulses of each group are sufficiently spaced that nobackscattered light from an earlier pulse remains in the fibre when alater pulse is launched. In FIG. 2H, backscattered light from at leastone, but not all earlier pulses of a group remains in the fibre when alater pulse is launched.

The sensor discussed above can be used to implement a number ofdifferent functionalities, separately or in various combinations, usinggroups of probe light pulses of different optical wavelengths such asthose illustrated in FIGS. 2B to 2H.

The probe light pulse repetition rate and therefore the interrogationrate of the sensing fibre 10 in the sensor system of FIG. 1, when usingprobe light pulses of a single wavelength as illustrated in FIG. 2A, islimited by the propagation delay for probe light backscattered at thedistal end of the sensor fibre to return to the launch end of thesensing fibre 10. Launching probe light pulses at a rate where thebackscattered light of successive probe light pulses overlaps at thedetector at best reduces the quality of the sensor output, for smalloverlaps prevents detection of useable backscatter signal from thedistal end of the sensing fibre, and for significant overlap renders thesensor largely inoperable.

To increase the interrogation rate, or for a vibration sensor samplingthe acoustic frequency range, beyond the maximum available from theNyquist limit set by the pulse repetition rate of probe light pulses forwhich backscatter does not overlap at the detector, multiple probe lightpulses each of a different optical wavelength can be used, for exampleas described above in connection with FIG. 2B to 2D. In this way,backscattered light from two or more probe light pulses can be detectedsimultaneously. Preferably the different optical wavelengths are chosenso that they can be multiplexed together so as to share the majority ofthe optical transmission and reception paths of the sensor, to minimisecomplexity and cost of the sensor, while also allowing the differentwavelengths to be separately received by different photodetectorcomponents.

If two different wavelengths are used then the probe light pulses of thetwo different wavelengths can be alternated in the launching scheme. Tomaximise the interrogation rate each probe light pulse of alternatingwavelength can be launched into the sensing fibre at about the same timeas or as soon as possible after a probe light pulse of the otherwavelength has reached the distal end 28 of the sensing fibre 10. Inthis way, probe pulses of one wavelength are launched at timesapproximately midway between those of the other wavelength, thusdoubling the interrogation rate of the sensing fibre. If the sensor isbeing used to detect acoustic vibrations then this scheme can double theacoustic bandwidth or frequency range compared to the case where asingle optical wavelength is used.

The interrogation rate can be extended further by launching probe lightpulses of more than two different wavelengths in a group of such pulses,again with the condition that backscattered probe light for successiveprobe light pulses of any particular wavelength does not overlap at thedetector. The interrogation rate can thus be increased in proportion tothe number of different wavelengths used.

FIG. 3 illustrates a scheme for deriving higher interrogation rate orsampling rate data, for example a higher acoustic bandwidth, fromdetected backscatter of interleaved probe light pulses of differentwavelengths. Steps 30 a and 30 b represent the simultaneous detection ofbackscattered light at wavelengths at λ_(a) and λ_(b) respectively.Signals from these detection steps are used in steps 32 a and 32 b toderive data relating to the detected backscatter of probe light of eachrespective wavelength at a plurality of positions along the sensingfibre, these positions being denoted by the box label numerals (1, 2, 3. . . ). For each of a plurality of positions or nominal locations alongthe sensing fibre, the data from the two wavelengths is time-interlacedat step 34, according to the order of the launch of the respective probelight pulses into the sensing fibre, to generate time series of the data36.

The sensor may be operated, for example, using phase-sensitive opticaltime domain reflectometry (OTDR) in which probe light pulses are usedwhich are each sufficiently coherent that the detected backscattersignal is dominated by self-interference between different parts of thesame pulse. Such techniques are discussed, for example, inWO2008/056143. The resulting Coherent Rayleigh backscatter thereforeleads to a temporal speckle pattern of interference fringes at thedetectors. The signals resulting from the detection steps may then be atime series of intensity of the detected temporal speckle patterns. Inorder to sense vibration as a function of position along the sensingfibre, the data relating to the backscatter may be a measure of changeover time of the temporal speckle pattern corresponding to a particularsensing fibre position, such as a simple difference between successivesamples corresponding to the same fibre position.

Using such techniques, vibration is sensed through the induced rapidsmall changes in strain and therefore refractive index of the sensingfibre 10. The signal resulting from a single probe light pulse beingbackscattered to the detector appears as a quasi-random oscillatorysignal of essentially static shape. From pulse to pulse the static shaperemains unchanged if the strain conditions and therefore refractiveindex are unchanged, but the shape changes to a lesser or greater extentas the sensing fibre is subjected to vibrationally induced strains of acorrespondingly smaller or larger magnitude. The detected optical signalmay be referred to as coherent Rayleigh noise, although for this type ofreflectometry the “noise” contains the signal of importance.

A notable feature of this reflectometry technique, and similartechniques based on interferometry, is that the intensity, or changes inintensity, of a repeated sampling of the detected optical signal mappingto a particular position or segment of the sensing fibre, is notintrinsically a linear function of the local forcing such as strain andits effect on refractive index at that fibre position. Rather, theresponse characteristics in the sampled signal to an increasing ordecreasing local physical forcing is periodic and of variable sign andscale factor. This occurs because of the interferometric nature of theoptical response.

The net result of the changing phase on the sum of all of thebackscattered components is typically to generate a sinusoidal variationin intensity of the detected backscatter as the relative phase ischanged linearly. Moreover, since the starting point of the intensitychange may be any point within the sinusoidal response, a givenfractional change in vibrationally induced strain can generate either apositive or negative fractional change in detected optical intensity,and with a scale factor that can vary between zero and some maximumvalue that depends on the interferometric fringe visibility within theoptical system. Since the lower limit of strain that can be detected bythe sensor is also limited by system noise arising from various sources,the dynamic range of the response of the sensor is constrained to liewithin a certain range of strain amplitude, and within an even morerestricted range if linear response characteristics are desired. Linearresponse characteristics are often desirable.

To illustrate this issue of non-linearity, for example when phasedisturbances significantly greater than π/2 radians are experienced at asingle sample point in the sensor fibre, FIG. 4A shows the relationshipbetween phase imbalance and output power from an ideal two-pathinterferometer. In the figure, the horizontal arrows above the graphindicate possible extents of variation in phase imbalance as the sensingfibre is influenced by weaker or stronger changes in strain, for exampledue to vibration. Variations with peak to peak amplitudes of π/2, 2π/3and 2π are shown around a phase bias of π/2 radians. Responsecharacteristics, as corresponding oscillatory variations of output powerfor each of these phase excursions, are shown in FIG. 4B. If the timeaxis is taken to be in milliseconds then these responses may correspondto vibrational signals at about 25 Hz. For the two smaller phasevariations, the output power variations are similar, confirming that theinterferometer is operating with a quasi-linear transfer functionbetween phase and power. However, for the largest phase variation, grossdistortion of the response characteristics, with a strong component attwice the driving frequency, is evident.

To address the issue of linearity or non-linearity of responsecharacteristics of the sensor to a physical parameter to be measured,the sensor of FIG. 1 may be operated by launching two or more probelight pulses of different pulse lengths, i.e. different physical lengthswithin the sensing fibre 10, and different lengths in terms of pulseduration from a laser source. If desired, some or all of the pulses ofdifferent pulse length may also be of different wavelength so thatbackscattered light from such pulses can be detected simultaneously. Theinterferometric path length imbalance for a probe light pulse isdependent upon the pulse length, so that reducing the relative length ofone probe light pulse has the effect of reducing the relativeinterferometric path imbalance for that pulse. If the detected waveformsfor the backscattered light for both pulse lengths are similar, as isfound between the π/3 and π/4 waveforms in FIG. 4B, then linearity ofthe sensor response characteristic is confirmed. If the detectedwaveforms for the backscattered light for both pulse lengths are ratherdifferent, as is found between the π and π/3 waveforms in FIG. 4B, thennon-linearity of the sensor response characteristics, and in particularof the longer length pulses, is confirmed. A measure of thenon-linearity of the response characteristics of at least one or otherof the two pulse lengths, typically the longer one, can be made bycomparing the two signal waveforms, for example to determine a measureof differences in shape.

If the measure of non-linearity exceeds a threshold then adjustments canbe made to the pulse lengths of the probe light pulses of one or bothpulses of the group in order to seek a more linear responsecharacteristic.

The described process of detecting non-linearity of response andadjusting probe light pulse length to compensate is illustrated in FIG.5, which shows selected elements of the sensor of FIG. 1 with furtheranalysis and control features. The probe light source 12 is shown toinclude two optical sources 50 a, 50 b operating to generate probe lightpulses of wavelength λ_(a) and λ_(b) respectively. Control of probelight wavelength within the probe light source 12 is indicated by pulsewavelength data elements 52 a and 52 b respectively. Control of probelight pulse length within the probe light source 12 is indicated bypulse length data elements 54 a and 54 b for the two wavelengthsrespectively. The generated probe light pulses are conditioned andcombined in source optics 56 before being launched into the sensingfibre 10 through optical circulator 58. The probe light pulses are eachsufficiently coherent and of narrow bandwidth that the detectedbackscatter signal is dominated by self interference between differentparts of the backscattered pulse, as discussed above in connection withphase-sensitive OTDR.

In this example, the probe light pulses of two (or more) differentwavelengths are launched simultaneously, or at least so that the pulsesoverlap as they travel along the sensing fibre 10. However, a widerpulse spacing could be used, at the risk of degrading the performance ofthe measure of non-linearity as the pulses of the group increasingly seedifferent states of the sensing fibre 10. For example, a difference inlaunch time between the pulses of not more than 2 microseconds (so thatthe two pulses would only see different vibrational signals at very highfrequencies beyond the acoustic, around 500 kHz), or not more than about400 meters in position of the respective pulses along the fibre could beused. In some embodiments, therefore, backscattered light from all ofthe two (or more) pulses used for the non-linearity detection may becoexistent or present in the sensing fibre at the same time.

In other embodiments, backscattered light from only some pulses in thegroup coexists within the fibre, and in other embodiments there is nosuch overlap such that backscatter from the pulses does not overlap atthe detector.

Probe light backscattered within the sensing fibre 10 is directed by thecirculator 58 to the detector 14 where, following wavelengthdemultiplexing and signal conditioning in detector optics 60, thebackscattered light is detected separately for each wavelength usingphotodetectors 62 a and 62 b respectively followed by digitisation at 63a,63 b. Signals corresponding to the detected backscattered light foreach wavelength are passed to the analyser 16. For some applications,these signals may be pre-processed in pre-processor 64 to form signalswhich relate more directly to the parameter to be measured and which aretherefore more suitable for comparison with each other to derive ameasure or indication of non-linearity. For example, for the temporalspeckle patterns or coherent Rayleigh noise signals arising fromRayleigh backscatter with sufficiently coherent probe light pulses,successive detected backscatter intensities corresponding to aparticular location along the sensing fibre for successive pulses of aparticular wavelength may be differenced or otherwise processed toderive a signal indicative of the degree or rate of change of thebackscatter signal from that location. This pre-processed signal, whichmay be more directly related to the vibration or other physicalparameter to be measured at that location, or alternatively a signal notprocessed in this way, is then passed to the comparator 66, where thesignals from the two wavelengths are compared. Essentially, thecomparator 66 determines and compares, directly or indirectly,properties of the response characteristics for each pulse length.Typical response characteristics which can be derived form the data mayinclude signal amplitude ratios between signals from differentinterrogation pulses, waveform shapes, and spectral components ofvibration signals.

Dependent upon results of that comparison a pulse length controllerelement 68 in the analyser sends control signals 20 to control the pulselength data elements 54 a and 54 b in the probe light source accordingto a pulse length control algorithm 70. The pre-processed signal mayalso be sent to further processing elements 69, for example to makedeterminations of the one or more physical parameters to be measured bythe sensor.

One way in which the pulse length control algorithm might operate tocontrol the pulse lengths would be to maintain a constant orproportional difference between the two pulse lengths controlled by dataelements 54 a and 54 b, for example a difference of 5 nanoseconds, or adifference of 50%, and to gradually increase the length of both, forexample by 1 nanosecond, or by 10%, between short periods of probe lightpulses of both wavelengths, where the monitoring periods are chosen tobe sufficiently long to cover a number of acoustic or vibration cycles.For example, typical monitoring periods might last for between one andfive seconds. If a measure of difference between the data derived fromthe two backscatter profiles exceeds a certain threshold, then insteadof increasing by 1 nanosecond or 10%, both pulse lengths would bedecreased, for example by 1 nanosecond or 10%. The measure of differencecould be an overall normalised acoustic intensity difference between thetwo signals at a location of interest, or alternatively, acousticspectral comparisons could be made. Of course, much more sophisticatedand adaptive schemes could be used.

The described technique of linearity control using comparison ofproperties of the response characteristics, and corresponding pulselength control may be used continuously or intermittently. For example,pulses of different lengths may be used continuously to monitor thenon-linearity of the response characteristics and adapt the pulse lengthvery rapidly, for example in response to sudden increases and decreasesin an acoustic signal to be detected, in which case the probe lightpulses used for detecting non-linearity may also used for detecting thephysical parameter to be measured. Alternatively, pulses of differentlengths and pulse length compensation may be used on a more intermittentbasis, with the sensor operating in other modes in between.

The technique of linearity control using signal comparison and pulselength control may be combined with the technique described above ofincreasing the interrogation rate of the sensing fibre by usinginterleaved pulses of different wavelengths. However, the technique ofincreasing interrogation rate is improved by more even spacing of thepulses of different wavelength, whereas the technique of linearitycontrol is improved by closer spacing of the pulses of different length.The technique of linearity control can, however, be used continuously insuch a context by adding a further pulse wavelength for launch at thesame time as one of the interleaved pulse wavelengths. Alternatively,these two techniques can be used sequentially for differing periodsaccording to a programmed sequence.

As for the technique of increasing interrogation rate described above,three or more different pulse lengths can also be used for linearitycontrol, for example to more rapidly and accurately determine an optimumpulse length for current use.

A physical parameter to be measured by the sensor such as strain orvibration may vary considerably both in time and in space along thesensing fibre, so that in general it won't be possible to select anoptimum pulse length which provides a response characteristic of optimumlinearity and sensitivity for all points along the sensing fibre.However, by launching an ensemble of probe light pulses of differentpulse lengths it is possible to extend the linear dynamic range of thesensor, whether or not a dynamic control of pulse length as discussedabove is also used.

FIG. 6 illustrates the sensor of FIG. 1 configured to interrogate thesensing fibre using phase sensitive OTDR as discussed above. Inparticular, the probe light source 12 is configured to launch a group offour probe light pulses, shown in figure box 80, each of differentwavelength λ₁, λ₂, λ₃, λ₄ into the sensing fibre 10. Each probe lightpulse also has a different length, increasing from the shortest lengthfor λ₁ to the longest length for λ₄, which together may be referred toas a pulse length ensemble. The longest length pulse provides the mostsensitivity to phase imbalances in the sensing fibre, but is thereforealso the most susceptible to non-linearity of the responsecharacteristic and is therefore unsuitable for detecting larger phaseimbalances which might result from large amplitude strains andvibrations. The shortest length pulse provides the least sensitivity tophase imbalances in the sensing fibre, but is therefore also the leastsusceptible to non-linearity of response for detecting larger phase biaschanges. Suitable durations for such a scheme are 10, 20, 50 and 100 nsfor each wavelength respectively. These pulse durations theoreticallyprovide approximate doubling of sensitivity between each successivewavelength and an overall linear dynamic range of ten times thatavailable from the shortest pulse used alone.

Aligned with the sensing fibre 10 in FIG. 6 is a plot 72 of a vibrationprofile against distance along the fibre. The plot shows levels ofvibration divided into four levels. The sensor is adapted to use thedetected signal from the λ₄ pulses, which have the longest duration, inregions of the fibre where the vibration levels are smallest, and thedetected signal from the λ₃, λ₂ and λ₁ pulses for regions of the fibrewith increasingly high levels of vibration. The selection of whichsignal to use for which region of the fibre is adaptive so that the bestsignal for each region is continuously or periodically selected, tomaintain an optimum level of sensitivity balanced against avoidingexcessive non-linearity in the detected signals. In particular, thesensor may explicitly or implicitly determine properties of the responsecharacteristics of the different pulse durations, and use suchproperties in the adaptive scheme.

More generally, the sensor may combine data from the four differentwavelengths, using weightings which are calculated according toproperties of the response characteristics. Of course, the weightingsmay be binary in the sense of selection of one or more wavelengths touse equally and rejection of the others, or they may include fractionalweightings where signals having poor response characteristic propertiesare given a lower weighting than signals having better responsecharacteristic properties.

The different wavelength and duration pulses of the pulse durationensemble may be launched into the sensing fibre at the same time, or atleast so that they overlap in spatial extent in the fibre, and thisbetter permits cross comparisons between the detected signals of theensemble to be made. However, in order to use the ensemble forfibre-position selection of pulse duration, simultaneously launching isnot required, although it may be advantageous for the pulses to belaunched close enough together either so that all pulses of the ensembleexist in the fibre at some point in time, or so that at least somebackscatter from all of the pulses of an ensemble coexists within thesensor fibre or reaches the detector at some point in time. In this way,the benefits of improving linearity of response can be maintainedwithout sacrificing the overall interrogation rate.

FIG. 7 illustrates in more detail how the pulse duration ensemble ofFIG. 6 can be put into effect in the sensor of FIG. 1, using some commonreference numerals also with FIG. 6 to denote like elements.Backscattered light from an ensemble of pulses of four differentwavelengths and correspondingly different pulse durations is received inthe detector 14 where it is processed and wavelength de-multiplexed bydetector optics 60 before being passed to detectors 62 a-62 d. Thedetected signals are digitised at A/D converter 63 before being passedto a pre-processor 73 where the signals are processed sufficiently to beable to detect non-linearity more effectively. For example, in the caseof vibration induced changes in coherent Rayleigh noise, acousticspectral profiles may be differenced. Linearity detector 75 monitors thepre-processed signals to determine which wavelength of the ensembleprovides the optimum properties of response characteristics, and hencethe optimum signal for particular corresponding regions of the sensingfibre, or to determine relative weights for the wavelengths of theensemble. This selection is passed to an ensemble selector 77 whichconstructs an assembled signal taking the optimum signals for particularregions of the sensing fibre, or forming a suitably weighted combinationof signals. The assembled signal may then be passed to furtherprocessing elements 69 as required, for example to make determinationsof the one or more physical parameters to be measured.

The linearity detector 75 may work, for example, by selecting theoptimum member of the pulse ensemble according to individual spectralcharacteristics, for example by applying an upper threshold to thepermitted incidence of higher acoustic harmonics characteristic ofnon-linearities of response in the signal for longer pulse durationscompared with shorter duration pulses. The power in higher frequencyacoustics, for example, may provide a suitable measure or property ofthe response characteristics for a particular pulse length. Anotherscheme of operation would be to accept the signal from the longestduration pulses which are of sufficient similarity to any or all shorterduration pulses, the shortest duration pulse being chosen where allpulses of the ensemble provide sufficiently different signals. In thiscase, the analysis of response characteristics is based on similaritybetween signals. Another scheme of operation would be to providefractional weights based on one or more determined properties of theresponse characteristics.

The pulse duration ensemble technique illustrated in FIGS. 6 and 7 maybe combined with the pulse duration control technique described inconnection with FIG. 5. In particular, the pulse duration controltechnique may be used to provide appropriate settings for the pulseensemble durations. The individual signals from the pulse durationensemble could also be processed in ways other than a simple selectionof an optimum pulse length for a particular region of the sensing fibreat a particular time, for example by using more complex statistical orsignal processing to combine the signals from the ensemble to derive anoutput signal such as a physical parameter to be measured with anoptimum signal to noise ratio.

The benefits of the different multi-wavelength, and optionally multipulse length interrogation techniques described above can be combined invarious other ways, both simultaneously and sequentially by choosing anappropriate mix of numbers of interrogation wavelengths and systemcontrol parameters. For example, even using only two wavelengths, avibration sensing system might be set up to first confirm a lineardetection regime using the pulse duration control or non-linearitydetection techniques, and then be switched into a interrogation rateenhancement mode using interleaved probe light pulses. Alternatively, athree-wavelength system might be configured to offer linearityconfirmation and enhanced detection bandwidth at the same time.

For all the techniques described above, it can be beneficial for thephase bias observed in the interference signal of the backscatteredprobe light for each location along the sensing fibre to be similaracross all the probe light pulse optical wavelengths, within a range of2π. This will not tend to occur unless controlled in some way, becausethe much larger absolute differences in total phase bias betweendifferent wavelengths will tend to give rise to arbitrary differenceswithin the 2π range. The differences in total phase bias can be manythousands of cycles for pulses with very different lengths in the rangeof several meters. When phase bias and imbalance, and differencesbetween phase biases and imbalances are discussed herein, differenceswithin the 2π range are generally meant, unless the context clearlyimplies absolute values including multiples of 2π.

FIG. 8a is similar to FIG. 4a discussed above, but with three verticaldashed lines representing phase biases for the undisturbed sensor fibre,for interference signals detected in backscattered probe light for threedifferent probe light wavelengths at a particular position along asensing optical fibre 10. In the situation of FIG. 8a , phase imbalancevariations about the phase bias for the λ₃ wavelength at 3π/2 give riseto optical signal changes which are of similar magnitude, but which varyin an opposite sense under changes in phase imbalance to those of the λ₁wavelength which has a phase bias at π/2. The λ₂ wavelength has a phasebias close to π, so phase imbalance variations give rise to a differentbehaviour again in the optical signal, which rises from a low level inboth directions.

FIG. 8b is similar to FIG. 8a , but the phase biases for wavelengths λ₁,λ₂ and λ₃ are close together within the 2π range, in this case beingcommonly aligned at about π/2 for some arbitrary position along thesensing fibre. The optical signal from all three wavelengths willtherefore behave in a similar fashion, subject, for example, to themagnitude of the phase imbalance response to a particular refractiveindex variation which depends in part on the length of each probe lightpulse as discussed above in connection with FIGS. 5, 6 and 7.

Note that although the phase bias in the backscatter interference signalfrom any given probe light pulse optical wavelength will vary throughoutthe 2π range along even short lengths of the sensor optical fibre forexample of only a few meters, the respective phase biases of probe lightpulses of multiple wavelengths will exhibit an approximately constantphase spacing, as demonstrated below. Therefore, if the phase biases fora group of pulses can be aligned or spaced in a particular way for oneposition in the sensor fibre, they will be approximately aligned orsimilarly spaced for all other positions along the sensor fibre.

If the behaviour illustrated in FIG. 8b can be achieved for a group ofpulses of different wavelengths, then comparison of sensor data betweenpulses of different durations as discussed in connection with FIGS. 5-6can be more easily made, because similar optical responsecharacteristics are expected, subject to pulses exceeding a certainduration tending to result in a more non-linear or distorted opticalresponse. Time series compiled by interleaving data from staggeredpulses of different optical wavelengths to increase the sensor samplingrate, as discussed in connection with FIG. 3, can also be improved,because the interleaved data from the two or more pulse wavelengths willbe expected to exhibit similar response characteristics to the changesin the sensor fibre. Of course, the effects achieved by controlling therelative phase bias between wavelengths to be small or close to zero canalso be achieved by relative phase biases of around π, with one of thesignals being inverted to compensate or equivalent processing beingused.

Although control of the phase bias in the interference signal from thebackscattered light for each probe pulse wavelength cannot offer uniformphase bias along the length of the sensing fibre, this is not needed toconstruct a useful sensor system, although it is beneficial to be ableto control relative phase bias across the set of probe pulse wavelengthsat any location along the fibre so that the response characteristicsacross all wavelengths are similar at any one position. However, if thesensor is adapted to control relative phase bias between wavelengths,another desirable system feature can be available. With two independentprobe light wavelengths, where there is controllable relative phase biasalong some or all of the sensor fibre, if the relative phase bias is setto π/2 radians as in the mathematical example set out below, and thedifferent wavelength pulses are launched sufficiently close together tobe affected by substantially the same detectable changes in the sensorfibre, then the variation in response characteristics with positionalong the sensor fibre can be reduced by processing the data from thedifferent wavelengths according to a vectorial method. This is because,as noted earlier, the output optical intensity of an ideal, two-pathinterferometer is proportional to one plus the cosine of theinstantaneous phase imbalance. Hence, if the relative phase bias for asecond probe wavelength compared to the first is π/2 radians, then thedetected optical signal for the second wavelength will be proportionalto one plus the sine of the instantaneous phase imbalance. When the zerooffset of these two signals is removed and the resulting data values areadded in quadrature, for example by being squared and added together,the resulting sum becomes independent of the actual phase bias at thatlocation, thus the variation of the optical response characteristicsobserved for any one wavelength, or for multiple wavelengths withaligned phase biases, is reduced or eliminated. Taking the square rootof the sum provides a linear measure of the amplitude of thedisturbances to the phase imbalance.

More generally, if the relative phase bias between two wavelengths isadjusted so that the resulting response characteristics are orthogonalin behaviour or have orthogonal components, then the phase imbalancedisturbance responses can be combined to provide a signal with reducedsensitivity to the individual response characteristics of eachwavelength.

Using two probe pulse wavelengths with a relative phase bias of aroundπ/2, absolute signal amplitude information is typically lost beyondphase imbalance disturbances of π radians. However, such information canbe preserved if a more advanced phase unwrapping algorithm is adopted.In one such method, a third interrogation channel is added, the relativephase bias of each channel is set to 2π/3 radians and the mathematicalprocessing scheme is designed accordingly. Mathematical schemes whichcould be used in such circumstances are discussed in B. V. Dorrio and J.L. Fernandez, “Phase evaluation methods in whole-field opticalmeasurement techniques”, Meas. Sci. Technol., vol 10, pp R33-R55 (1999).

Accordingly, embodiments of the invention provide for misaligning thephase biases of two or more probe light pulse wavelengths by controlledamounts. FIG. 8c illustrates an arrangement of phase bias of multipleprobe light wavelengths. The solid vertical lines show phase biases fora first arbitrary position along the sensing fibre at about π/2 and πfor wavelengths λ₁ and λ₂, with a relative separation of about π/2. Thedashed vertical lines show phase bias for a second arbitrary positionalong the sensor fibre at about 3π/4 and 5π/4, again with a phase biasseparation of about π/2. As discussed above, for each of these twoarbitrary positions along the sensor fibre, and indeed any otherarbitrary position where the separation between the phase bias is aboutπ/2, variations in the optical signal for the two wavelengths caused bydisturbances in the phase imbalance can be added vectorially to yield asummed optical response which under ideal circumstances is approximatelyconstant along the sensing fibre. Such a vectorially summed responsewill generally provide a more consistent response characteristic tochanges in the phase imbalance than the signal from any single probelight wavelength taken alone.

Although it would be useful to control the relative phase bias betweentwo probe light pulse wavelengths to be exactly zero, π, or π/2 radiansas discussed above, some variation about these values is acceptablebefore the described techniques become ineffective. For example,controlling the relative phase bias of two pulses of differentwavelength to within ±π/4 or between 3π/4 and 5π/4 radians, or morepreferably π/8 radians or between 7π/8 and 9π/8, where alignment isintended, may give adequate results. Similarly, controlling the relativephase bias of two pulses of different wavelength to between π/4 and 3π/4radians or more preferably between 3π/8 and 5π/8 radians, whereorthogonality is intended, may give adequate results.

The utility of schemes which control the relative phase bias betweendifferent pulse wavelengths can be understood as follows. Consider asensing system in which the pulse light wavelengths are closely spacedat around 1500 nm, and the effective path imbalance and refractive indexof the sensing fibre are about 1 m and 1.5 respectively. The relativephase given by equation (1) above is then about 2π×10⁶ radians. If thewavelength of a probe light pulse is then changed very slightly toeffect a change of Δφ_(bref) in phase of only a few cycles or less, thenthe fractional change in the absolute phase bias is extremely small. Thefollowing expressions give an example where the desired change in phasebias is π/2 radians. For this case, the required difference between thetwo probe light pulse wavelengths can be calculated by substituting thetwo interrogation conditions into equation (1) above to yield:Δφ_(bref)=2πn _(e) d(1/λ₂−1/λ₁)=π/2  (2)

The above expression neglects chromatic dispersion in the sensor fibre.The change in wavelength Δλ needed to produce the required π/2 phasechange is then given by:Δλ=λ₁λ₂/(4n _(e) d)  (3)

For this example, the required value of Δλ is approximately 0.37picometers. Now consider a different part of the sensing fibre where therefractive index is slightly different from n_(e), due to non-uniformityor some perturbation of the fibre. If the effective refractive index ofthis alternative section of the fibre is n_(es), then the differenceΔφ_(s) between the phase biases at the two probe light pulse wavelengthsis given by an expression similar to the first part of equation (2),with n_(e) replaced by n_(es). Thus we have:Δφ_(s)=2πdn _(es)Δλ/λ₁λ₂  (4)

Substituting in the value of Δλ from equation (3), we find that thechange in wavelength produces a change Δφ_(s) in phase bias at thesensing section ofΔφ_(s)=(π/2)(n _(es) /n _(e))  (5)

This expression indicates that, given two probe pulse wavelengths with arelative phase bias of π/2 in one section of sensor fibre having arefractive index of n_(e), the relative phase bias in another sectionhaving a refractive index of n_(es) is only different by a ratio of therefractive indices. Since the effective refractive index of typicalsingle mode fibres is normally very uniform after manufacture and cannotbe changed by more than a very small percentage by typical sensingsituations, equation (5) shows that a desired relative phase bias can beadequately maintained between probe light pulses of two or morewavelengths along the whole of the sensing fibre.

An example scheme in which the relative phase bias between two or moreprobe light pulses of different wavelengths can be regulated orcontrolled as discussed above is to apply a predetermined oscillatoryforcing, for example at an acoustic frequency, to a reference section ofthe sensor fibre 10. A corresponding oscillatory signal is then detectedin the backscattered light in respect of each of the different probepulse wavelengths for the reference section. These oscillatory signalscan then be compared, and the pulse wavelengths adjusted slightly untilthe detected oscillations at the frequency of the forcing display thedesired amplitude difference or other suitable characteristics.

For example, to obtain a relative phase bias of about π/2 between twopulse wavelengths, the wavelengths may be adjusted until the oscillatorysignal at the frequency of the forcing for one probe pulse wavelength isminimised or close to zero, and the corresponding signal for the otherwavelength is at a maximum. Similarly, to obtain a relative phase biasof zero, the two wavelengths may be adjusted until the oscillatorysignals are both either minimised or zero, or both maximised.

An arrangement suitable for putting such schemes into effect isillustrated in FIG. 9. As already shown in FIG. 5, the probe lightsource 12 comprises at least two laser sources 50 a, 50 b. The lengthsof probe light pulses may be controlled, if desired, in accordance withpulse length data elements 54 a, 54 b, under the control of pulse lengthcontroller element or function 68. The wavelength of probe light pulsesis controlled in accordance with pulse wavelength data elements 52 a. 52b, under the control of a pulse wavelength controller element orfunction 90. Either or both of the pulse length controller and pulsewavelength controller elements or functions may be integrated into orcombined within the analyser 16, or may be carried out elsewhere.

A relative phase bias calibrator 92 is provided to enable the pulsewavelength controller 90 to detect and adjust phase biases betweendifferent wavelengths. In the arrangement illustrated in FIG. 9 therelative phase bias calibrator is arranged to apply an acoustic forcingof predetermined frequency to a length of the sensing fibre, althoughthe forcing could be applied to a parallel length of similar fibre ifrequired, with the similar fibre suitably coupled to the probe lightsource 12 and detector 14. In an example described in more detail belowthe forcing is applied by wrapping a suitable length of the sensingfibre 10 around a piezoelectric drum coupled to a driving circuit. Thecalibrator 92 may be under the control of the analyser 16 if required,for example to turn the forcing signal on and off as needed, or toadjust the acoustic frequency of the forcing.

The pulse wavelength controller element 90 detects, in data derived fromthe light backscattered from the probe light pulses, the oscillatorysignal resulting from the forcing applied by the calibrator, and makesadjustments to the pulse wavelength data elements 52 a and 52 b toregulate the wavelengths of the probe light pulses so as to achieve therequired relative phase bias.

FIG. 10 shows some suitable details of optics and electronics forputting into effect the distributed optical fibre sensor of FIG. 1. Thevarious data processing and control schemes discussed above may beimplemented within this framework.

Two separate optical sources 50 a and 50 b are shown within the probelight source 12, each optical source emitting a narrow band ofwavelengths centred at λ_(a) and λ_(b) respectively. It is alternativelypossible to use a singe, wavelength switched source, or a combination ofsources with switchable and fixed wavelengths. Since two separateoptical sources are shown in this embodiment, a wavelength combinercomponent 114 is required to route the two signal wavelengths onto acommon optical path. If a wavelength switched source were to be usedalone, then this component would not be required. For convenience ofimplementation, the wavelengths λ_(a) and λ_(b) used in the system wouldlie within the operating band of typical erbium-doped fibre amplifiers,between 1528 nm to 1562 nm, and the optical sources would bedistributed-feedback laser diodes.

Once combined, the two signal wavelengths are then fed through anoptical conditioning chain 116 whose function is to amplify the light tosuitable power level and to provide optical filtering to avoid thedeleterious effects of amplified spontaneous emission (ASE) from theamplifier elements. Typically, peak powers of the order of 1 W might bedelivered to the sensing fibre, and the ASE suppression bandwidth mightbe ˜0.2 nm. Light emerging from the optical conditioning chain isdirected to an optical circulator 118 that serves to route probe lightfrom the probe light source 12 into the sensing fibre 10, and light fromthe sensing fibre 10 into the detector 14.

After the circulator 18, a coil 120 of the sensing fibre 10 wrappedaround a strain transducer 122 is used to provide a controllable,repeatable, periodic strain to the fibre, although a length of opticalfibre separate to the sensing fibre could instead be used for thispurpose. The strain transducer 122 may preferably be a piezoelectriccylinder of radius large enough to avoid bend loss in the fibre coil.This transducer is driven at a convenient acoustic frequency by a driverunit 124. The driver unit 124 may run autonomously or may be controlledfrom analyser 16. The fibre coil 120 comprises a length of fibre that isat least long enough to contain the maximum pulse length generated bythe probe light source 12. In typical systems, this maximum pulse lengthmight be 50 m. The function of the strain transduction system comprisingthe fibre coil 120, transducer 122 and driver 124 forms the relativephase imbalance calibrator 92 discussed earlier in connection with FIG.9.

Backscattered light returning from the sensing fibre through thecirculator 118 is directed into the detector 14, and in particular intoan optical signal conditioning chain 120. This chain contains furtheramplification and filtering components required to increase the receivedsignal powers to levels suitable for low-noise detection. Followingpassage through the signal conditioning chain 120, the two signalwavelengths λ_(a) and λ_(b) are separated by the wavelengthdemultiplexing component 122. After separation, the two signalwavelengths λ_(a) and λ_(b) are each further filtered to a narrow bandusing components 124 a, 124 b and 126 a, 126 b respectively. In thisembodiment, the narrow band filters are fibre Bragg gratings withapproximately 80 pm reflection bandwidth. Finally, each wavelength isreceived by its own photodetector, 130 a and 130 b respectively.Conveniently, PIN photodiodes may be used for this purpose.

The signals from each photodetector 130 a, 130 b are digitized by thedata acquisition unit 134 and fed to the analyser 16, which controls theoptical sources 110 a and 110 b via driver circuits 150 a and 150 b.Apart from providing accurately timed electrical pulses to the opticalsources to control probe light pulse timing and length, these drivercircuits also serve to fine-tune the wavelength of the optical sourcesfor precise control of relative phase bias between the probe lightpulses in a group. This can be achieved, for example, by control oflaser temperature. In possible alternative embodiments, fine-tuning ofthe centre wavelength of different probe light pulses might beaccomplished by controlled filtering either before the unconditionedprobe light enters the wavelength combiner 114 or after leaving thewavelength demultiplexer 122. In the latter case, the centre wavelengthof one or both of the fibre Bragg gratings could be thermally tuned. Inanother possible embodiment, fine control of wavelength mightalternatively be achieved by phase or frequency modulation of lightusing a radio frequency optical modulator together with appropriatefiltering.

Based on analysis of the backscattered probe light, the analyser 16provides control signals to the driver circuits 150 a, 150 b to put intoeffect the various aspects of the invention described elsewhere in thisdocument, including control of probe light pulse length and probe pulsewavelength as required.

Although various embodiments of the present invention have beendescribed above, it should be understood that they have been presentedby way of example only, and not limitation. It will be understood bythose skilled in the relevant art(s) that various changes in form anddetails may be made therein without departing from the spirit and scopeof the invention as defined in the appended claims. For example, inarrangements where it is not required to detect backscattered light fromat least two wavelengths from a group of pulses simultaneously, forexample in variations on the arrangements of FIGS. 5, 7, 9 and 10, asingle photodetector may be used, with no separation by wavelength inthe signal conditioning chain then being required. Similarly, a singletunable optical source may be used if desired to simplify the sensorwhere appropriate.

The invention claimed is:
 1. A distributed optical fibre sensor fordetermining at least one physical parameter as a function of positionalong a sensing fibre using optical time domain reflectometry, fromproperties of probe light backscattered within the sensing fibre, thesensor comprising: a probe light source arranged to launch probe lightpulses into the sensing fibre, the probe light pulses includingsuccessive groups each of two or more probe light pulses havingdifferent optical wavelengths to each other and being launched such thatbackscattered light from all the pulses of each group coexists withinthe sensing fibre; a detector arranged to detect probe lightbackscattered in said sensing fibre, including separately andsimultaneously detecting light of each of said different wavelengths;and an analyser arranged to determine said parameter as a function ofposition along the sensing fibre from said detected backscattered probelight.
 2. The distributed optical fibre sensor of claim 1 wherein thedetector comprises a plurality of photodetector elements, and the sensoris arranged such that each photodetector element detects light of adifferent one of the probe light pulses in each group.
 3. Thedistributed optical fibre sensor of claim 1 wherein the probe lightsource is arranged to interleave launching of the probe light pulses ofdifferent wavelengths, and the analyser is arranged to construct, for aplurality of positions along the sensing fibre, time series of saidparameter by interleaving data derived from the detected backscatter ofprobe light for each of said different wavelengths.
 4. The distributedoptical fibre sensor of claim 3 wherein the sensor is an acoustic sensorarranged to detect acoustic vibration as a function of position alongthe sensing fibre from the constructed time series, the saidinterleaving providing an enhancement of the acoustic range of thesensor.
 5. The distributed optical fibre sensor of claim 1 wherein allthe probe light pulses of each group coexist within the sensor fibre. 6.The distributed optical fibre sensor of claim 5 wherein all the probelight pulses of each group overlap within the sensor fibre, or arelaunched by the probe light source at the same time, or are launchedwithin 2 microseconds of each other.
 7. The distributed optical fibresensor of claim 1 wherein the different optical wavelengths of the probelight pulses in a group lie within a wavelength band of 1 nanometerwidth.
 8. The distributed optical fibre sensor of claim 1 wherein theparameter is a measure of vibration.
 9. The distributed optical fibresensor of claim 1 wherein the parameter is indicative of anenvironmental influence to which the sensor fibre is responsive.
 10. Thedistributed optical fibre sensor of claim 1 wherein the sensor is aninterferometric sensor arranged to: launch said probe light pulses suchthat interference signals from each wavelength are separately detectablein the backscattered light; separately detect said interference signalsin the backscattered light of each optical wavelength; and determinesaid parameter as a function of position along the sensing fibre fromsaid separately detected interference signals of one or more of saidwavelengths.
 11. The distributed optical fibre sensor of claim 10 wherethe sensor is arranged such that the detector detects coherent Rayleighnoise at each of said different wavelengths, and the analyser determinessaid parameter from properties of said coherent Rayleigh noise of one ormore of said wavelengths.
 12. The distributed optical fibre sensor ofclaim 1 wherein the sensor is arranged to adjust one or more of saidplurality of different wavelengths to control relative phase bias orbiases between interferometric signals in the backscattered probe lightarising from respective probe light pulses of said differentwavelengths.
 13. A method of operating a distributed optical fibresensor to determine at least one parameter as a function of positionalong a sensing fibre using optical time domain reflectometry, fromproperties of probe light backscattered within the sensing fibre, themethod comprising: launching probe light pulses into the sensing fibre,the probe light pulses including successive groups each of two or moreprobe light pulses having different optical wavelengths to each other;detecting probe light backscattered in said sensing fibre, includingseparately detecting light of each of said different wavelengths; anddetermining said parameter as a function of position along the sensingfibre from said detected backscattered probe light of one or more ofsaid wavelengths, wherein the step of launching comprises launching theprobe light pulses such that backscattered light from all the pulses ofeach group coexists within the sensing fibre, and the step of detectingcomprises separately and simultaneously detecting light of each of saiddifferent wavelengths.
 14. The method of claim 13, further comprisingautomatically adjusting one or more of said plurality of differentwavelengths to control the relative phase bias between interferometricbackscatter of said different wavelengths.
 15. The method of claim 13further comprising interleaving launching of the probe light pulses ofdifferent wavelengths, and constructing, for a plurality of positionsalong the sensing fibre, time series of said physical parameter byinterleaving data derived from the detected backscatter of probe lightfor each of said different wavelengths.
 16. The method of claim 15wherein the sensor is an acoustic sensor arranged to detect acousticvibration as a function of position along a sensing fibre, theinterleaving providing an enhancement of the acoustic range of thesensor.
 17. The method of claim 13 wherein the parameter is indicativeof an environmental influence to which the sensor fibre is responsive.18. The method of claim 17 wherein the parameter is determined from itseffect on the backscattering of said probe light giving rise to changesin interference signals detected in the backscattered probe light. 19.The method of claim 17 wherein the parameter is a measure of vibration.20. The method claim 13 wherein all the probe light pulses of each groupcoexist within the sensor fibre.
 21. The method of claim 20 wherein allthe probe light pulses of each group overlap within the sensor fibre, orare launched by the probe light source at the same time, or are launchedwithin 2 microseconds of each other.
 22. The method of claim 13 whereinthe different optical wavelengths of the probe light pulses in a grouplie within a wavelength band of 1 nanometer width.
 23. The method ofclaim 13 wherein the sensor is an interferometric sensor arranged to:launch said probe light pulses such that interference signals from eachwavelength are separately detectable in the backscattered light;separately detect said interference signals in the backscattered lightof each optical wavelength; and determine said parameter as a functionof position along the sensing fibre from said separately detectedinterference signals of one or more of said wavelengths.
 24. The methodof claim 13 wherein the step of detecting comprises detecting coherentRayleigh noise at each of said different wavelengths, and the parameteris determined from said properties of said coherent Rayleigh noise ofone or more of said wavelengths.